Forming screen for use in a paper machine

ABSTRACT

The invention relates to a forming screen for use in the forming section of a paper machine, comprising a paper and a machine side as well as longitudinal threads extending in the intended running direction and binding cross threads extending transversely thereto, wherein the longitudinal threads are divided in the ratio of the number of threads of 1:1 in a longitudinal thread layer on the machine side and a longitudinal thread layer on the paper side, and the binding cross threads bind in both longitudinal thread layers, which is characterized in that the forming screen has a fiber support index (FSI) which is in the range of 230 to 250 and has an air permeability which is in the range of 5700 to 10600 m 3 /m 2 /h.

CROSS-REFERENCE TO RELATED APPLICATIONS AND CLAIM TO PRIORITY

This application is related to application no. 07 023 458.8, filed Dec.4, 2007 in the European Patent Office, the disclosure of which isincorporated by reference and to which priority is claimed.

FIELD OF THE INVENTION

The invention relates to a forming screen for use in the forming sectionof a paper machine, comprising a paper side and a machine side as wellas longitudinal threads extending in the intended running direction andcross threads extending transversely thereto, whereby the longitudinalthreads are divided in the ratio of the number of threads of 1:1 into alongitudinal thread layer on the machine side and a longitudinal threadlayer on the paper side and the binding cross threads bind in bothlongitudinal thread layers.

BACKGROUND OF THE INVENTION

Forming screens are used in the sheet forming region of a paper machine.These are endless web bands of several ten meters in length and severalmeters in width which are guided over rollers in such a way that theyform an essentially even surface on the top. At the start of the run, afiber pulp is applied to said surface which is drained downward throughthe forming screen, while the fibers accumulate on the surface of theforming screen and in this way form a paper web. At the end of theforming screen, the paper web, which is still very sensitive, is thentaken over by a press conveyor and then conveyed to the pressing partfor further draining and to the drying part for thermal drying.

A number of various web constructions have been proposed for the webstructure of forming screens. The present invention relates to a type ofweb in which the longitudinal threads are warp threads and are arrangedin two longitudinal thread layers with longitudinal threads arrangedabove one another in the ratio of the number of threads of 1:1. Formingscreens of this type are, for example, known from EP 1 362 142 B1, WO2004/094719 A1 and WO 2005/001197 A1. The longitudinal thread layers arejoined by means of binding cross threads which bind both in the machineside and in the paper side of the longitudinal thread layer. In additionto these binding cross threads, further cross threads can also bepresent which bind only in the longitudinal thread layer on the paperside and/or only in the longitudinal thread layer on the machine side.The aforementioned type of web is distinguished by a good compromisebetween the contradictory requirements for good fiber support on the onehand—EP 1 362 142 B1 notes an FSI value of 185 and/or 188 for this—and,on the other hand, high dewatering efficiency.

Basically, forming screens are today made of plastic threads formed frommonofilaments. In this case, high-strength plastics prevail more andmore in order to be able, at the same strength, to realize smallerthread diameters, in particular in longitudinal direction, and thus alower fill degree of longitudinal threads, and consequently a higherpermeability (see WO 03/046277 A1 in a generically foreign formingscreen having two cross thread layers and binding warp threads). In thistype of web, the fill degree of longitudinal threads is usually at atleast 100%, partially much higher (see WP 2006/034576 A1; WO 01/27385A1; EP 0 998 607 B1; WO 2004/013410 A1), that is in spite of the use ofhigh-strength longitudinal or warp threads. In part, lower longitudinalthread fill degree are also noted (see U.S. Pat. No. 4,314,589; EP 0010311; U.S. Pat. No. 4,379,735), whereby, however, this type of web is notcomparable in this respect with the generic type of web. In genericallyforeign types of webs, air permeability of between 7,500 to 10,500m³/m²/h (WO 2004/013410 A1) or 3,500 to 8,200 m³/m²/h (see WO 01/27385A1) are attained.

Despite the advances achieved in the interim, as before, there is needfor improvement in order to increase the efficiency of forming screenswith respect to their degree of fiber support on the one hand anddewatering efficiency on the other hand, and thereby obtain a paper webat the end of the forming screen which has a lower degree of moisturecontent and a higher fiber density and consequently strength. The resultof improvements of this type are that the subsequent sections of thepaper machine are stress-relieved or high degrees of dryness areattained and paper tears prevented.

Consequently, the object of the invention is to design a forming screenin such a way that a paper web is obtained at the end of the formingscreen which has a higher strength and a lower degree of moisturecontent.

SUMMARY OF THE INVENTION

According to the invention, this object is solved by a forming screenwhich has a fiber support index (FSI) which is in the range of 230 to250 and has an air permeability which is—always measured at 100 Pa—inthe range of 5700 to 10600 m³/m²/h (correspondingly 350-650 cfm,measured at 127 Pa), preferably in the range of 6500 to 8200 m³/m²/h(correspondingly 400-500 cfm, measured at 127 Pa). Thus, the basic ideaof the invention is to design the web structure of the forming screen insuch a manner that a superior fiber support is obtained at constantstrength and in spite of very high air permeability and with thereforedewatering effect, so that only a small portion of fibers is lost duringthe water draining and a paper web of realtively good strength, which isuniform over the surface, is formed. In this case, it should be takeninto account that, when calculating the fiber support index, not onlythe number of longitudinal and cross threads per cm comes into it, butalso two coefficients which depend on the type of binding of thelongitudinal and cross threads and their contributions to the fibersupport, i.e. the fiber support index indirectly also says somethingabout the web structure of the forming screen.

The fiber support index is calculated according to the publication“Approved Standard Measuring Method” of the Papier-machine ClothingAssociation, 19, rue de la République, 45000 Orléans/France, of June2004 according to the following formula:

FSI=1,693(a×Nm+2×b×Nc),

wherein

-   -   a=coefficient for the support contribution of the longitudinal        threads;    -   Nm=number of longitudinal threads per cm in transverse direction        on the paper side of the forming screen;    -   b=coefficient for the support contribution of the cross threads;    -   Nc=number of cross threads per cm in longitudinal direction on        the paper side of the forming screen.

The coefficients a and b depend on the weave and can be found in a tablewhich is contained in the aforementioned document.

The air permeability is also measured according to the particulars inthe aforementioned document using a differential pressure of 100 or 127Pa (see above).

In an embodiment of the invention, it is provided that the distributionof longitudinal and cross threads is obtained in such a way that thereare 1200 to 1600 flow-through passages per cm surface of the formingscreen on the paper side. Thread run and thread structure should therebybe such that the flow-through passages are rectangular in shape with aratio of extension in longitudinal direction to the extension intransverse direction of 1:1,8 to 1:3,4. In this way, a good dewateringeffect results at high fiber retention.

According to a further feature of the invention, it is provided that thefill degree of the longitudinal threads in the longitudinal thread layeron the paper side is maximum 32% in order to obtain a high degree ofopenness in this longitudinal thread layer. On the whole, the fillcapacity of the longitudinal threads should be maximum 90%, preferablymaximum 85%, and thus clearly lower than in the previously known formingscreens of this type.

According to a further feature of the invention, it is provided that thediameter of the longitudinal threads, at least in the longitudinalthread layer on the paper side, is in the range of 0.08 to 0.12 mm,preferably 0.11 mm, and the diameter of the longitudinal threads in thelongitudinal thread layer on the machine side in the range of 0.15 or0.25 mm. Due to this comparatively small diameter of the longitudinalthreads, in particular in the longitudinal thread layer on the paperside, a small fill degree can be realized yet at a high specific threadnumber, i.e. a relatively open web is obtained which nevertheless stilloffers a good fiber retention. In order to be able to meet high strengthrequirements despite the small diameters, the longitudinal threadsshould consist of a high-strength plastic, e.g. of PET (polyethyleneterephthalate) having an intrinsic viscosity (IV) of at least 0.8 dl/gand a modulus of elasticity of at least 10 N/mm ² and/or of PEN(polyethylene napththalate).

A good compromise between dewatering effect, on the one hand, and fiberretention, on the other hand, is obtained when the number oflongitudinal threads is in the range of 50 to 60 threads per cm intransverse direction. With respect to the number of cross threads, itshould be in the range of 100 to 110 threads per cm in longitudinaldirection.

In principle, a forming screen according to the invention—with respectto cross threads—can be realized only with binding cross threads, i.e.with such (first) cross threads which bind in both longitudinal threadlayers and in this way make a connection between these layers. However,the fiber retention can be improved if there are, in addition, two crossthreads which are only bound in the longitudinal thread layer on thepaper side and alternate advantageously with the binding cross threadsin a specific regular way. Preferably, the second cross thread takenseparately should form a plain weave with the longitudinal threads ofthe longitudinal thread layer on the paper side.

According to a further feature of the invention, it is provided that thebinding cross threads are arranged adjacent to one another to formgroups of at least two binding cross threads, one of the binding crossthreads binding in the longitudinal thread layer on the machine sidewhen the other binding longitudinal thread or the other binding crossthreads bind(s) in the cross thread layer on the paper side. Within thegroups of adjacent binding cross threads, therefore, the binding crossthreads should extend so as to be staggered vis-à-vis one another intransverse direction of the forming screen. However, they each have thesame repeating, i.e. bind in the same way in the longitudinal threadlayer both on the machine side and on the paper side. Preferably, therespectively adjacent binding cross threads should bind in thelongitudinal thread layer with their repeating staggered in such a waythat their binding patterns complement one another without a gap, i.e.along their extension, preferably such that the complementary bindingsform a pattern on the paper side which corresponds to that of theoptionally present second cross threads, for example, also produce atype of plain weave.

It is also advantageous for the fiber support if the binding crossthreads bind in the longitudinal thread layer on the machine side withina connection with fewer longitudinal threads than in the longitudinalthread layer on the paper side, preferably with not more than twolongitudinal threads in the longitudinal thread layer on the machineside. In the longitudinal thread layer on the paper side, the bindingcross threads bind within a repeating, advantageously with more thanthree longitudinal threads.

According to a further feature of the invention, it is provided thatthere are third cross threads which bind only in the longitudinal threadlayer on the machine side. In particular, they are to be used to providewearing volume on the machine side of the forming screen for the purposeof protecting the tensile-stressed longitudinal threads in thelongitudinal thread layer on the machine side. For this purpose, it isadvantageous if the third cross threads float on the outer side of thelongitudinal thread layer on the machine side over several longitudinalthreads, whereby the number of these longitudinal thread is alwaysgreater than the number of longitudinal threads with which the thirdcross threads bind between two floatings. Preferably, the floatingsshould go over at least three, better still at least five longitudinalthreads. A binding with a single longitudinal thread of the longitudinalthread layer on the machine side reaches between the floatings.

All range data, both in the description and in the claims, also refer tothose values and thus also disclose those which lie between thethreshold values given for the ranges. Therefore, they are notexplicitly noted.

BRIEF DESCRIPTION OF THE FIGURE(S)

The invention is illustrated in greater detail in the drawing withreference to an embodiment. It shows, in cross section, i.e.transversely to the intended running direction, a section of the formingscreen 1 according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The forming screen 1 is a two-layer web having an upper longitudinalthread layer 2 on the paper side and a lower longitudinal thread layer 3on the machine side. Both longitudinal thread layers 2, 3 are formed bylongitudinal or warp threads—for example, designated with 4 or 5,respectively—in the ratio of the number of threads of 1:1, whereby alongitudinal thread 4 of the longitudinal thread layer 2 on the paperside is arranged exactly above a longitudinal thread 5 in thelongitudinal thread layer 3 on the machine side, i.e. the longitudinalthreads 4, 5 each lie in pairs above one another.

The diameter of the longitudinal threads 4 in the longitudinal threadlayer 2 on the paper side is 0.11 mm and the diameter of thelongitudinal threads 5 in the longitudinal thread layer 3 on the machineside is 0.18 mm. They consist of PEN. The number of longitudinal threadsis 58 per cm in transverse direction.

Second cross threads 6, 7 are bound in the longitudinal thread layer 2on the paper side while forming a plain weave with the longitudinalthreads 4 of this longitudinal thread layer 2. Vertically to the planeof the drawing, adjacent second cross threads 6, 7 follow in each case.The second cross threads 6, 7 consist of PET and they have a diameter of0.11 mm.

Third cross threads are only bound in the longitudinal thread layer 3 onthe machine side, only one of said third cross threads can be seen here.The third cross threads 8 each form floatings (designated with 9 by wayof example) going over five longitudinal threads 5 on the machine sideof the forming screen 1 and then each binds with a single longitudinalthread 5. The floatings 9 represent abrasives for protecting thelongitudinal thread layer 3 on the machine side which is under greattensile stress. The diameter of the third longitudinal threads is 0.22mm. The material can be PET and/or PAM.

The longitudinal thread layers 2, 3 are joined by pairs of two bindingcross threads 10, 11 each adjacent to one another in longitudinaldirection. They have a diameter of 0.11 mm and consist of PET. They bindin the longitudinal thread layer 2 on the paper side one after theother, alternatively on the top and bottom with longitudinal threads 4,then extend between the two longitudinal thread layers 2, 3 over threelongitudinal threads 4 or 5 and then bind with a single longitudinalthread 5 in the longitudinal thread layer 3 on the machine side beforethey then again float between the two longitudinal thread layers 2 and2. They each have the same repeating, however, they are staggeredvis-à-vis one another in such a way in transverse direction , i.e. intheir longitudinal direction, that the binding patterns in thelongitudinal thread layer 2 on the paper side complement one another, sothat they then together form a fabric pattern similar to the secondcross threads 6, 7. In this way, an almost uniform plain weave patternis produced on the paper side of the forming screen 1, said plain weavepattern offering a high fiber retention.

The fill degree of the longitudinal thread layer 2 on the paper side is31%, the total fill degree of the longitudinal threads 4, 5 is 82%. Thenumber of cross threads is 108 per cm in longitudinal direction of theforming screen 1. This results in a high openness and thus a gooddewatering effect in spite of a high fiber retention.

1. A forming screen (1) for use in the forming section of a papermachine, comprising a paper and a machine side as well as longitudinalthreads (4, 5) extending in the intended running direction and bindingcross threads (10, 11) extending transversely thereto, wherein thelongitudinal threads (4, 5) are divided in the ratio of the number ofthreads of 1:1 in a longitudinal thread layer (3) on the machine sideand a longitudinal thread layer (2) on the paper side, and the bindingcross threads (10, 11) bind in both longitudinal thread layers (2, 3),characterized in that the forming screen (1) has a fiber support index(FSI) which is in the range of 230 to 250 and an air permeability whichis in the range of 5700 to 10600 m³/m²/h.
 2. The forming screenaccording to claim 1, characterized in that the air permeability is inthe range of 6500 to 8200 m³/m²/h.
 3. The forming screen according toclaim 1, characterized in that the forming screen (1) on the paper sidehas 1200 to 1600 flow-through openings per cm² surface of the formingscreen (1).
 4. The forming screen according to claim 1, characterized inthat the flow-through openings are rectangular in shape with a ratio ofthe extension in running direction to the extension in transversedirection of 1:1,8 to 1:3,4.
 5. The forming screen according to claim 1,characterized in that the fill degree of the longitudinal threads (4) inthe longitudinal thread layer (2) on the paper side is maximum 32%. 6.The forming screen according to claim 1, characterized in that the totalfill degree of all longitudinal threads (4, 5) is maximum 90%,preferably maximum 85%.
 7. The forming screen according to claim 1,characterized in that the diameter of the longitudinal threads (4, 5),at least in the longitudinal thread layer (2) on the paper side, is inthe range of 0.08 to 0.12 mm.
 8. The forming screen according to claim1, characterized in that the diameter of the longitudinal threads (5) inthe longitudinal thread layer (3) on the machine side is in the range of0.15 to 0.25 mm.
 9. The forming screen according to claim 1,characterized in that at least a part of the longitudinal threads (4, 5)consist of PET with an intrinsic viscosity of at least 0.8 dl/g and anelasticity module of at least 10 N/mm².
 10. The forming screen accordingto claim 1, characterized in that at least a part of the longitudinalthreads (4, 5) consist of PEN.
 11. The forming screen according to claim1, characterized in that the number of longitudinal threads (4, 5) arein the range of 50 to 66 threads per cm in transverse direction.
 12. Theforming screen according to claim 1, characterized in that the number ofcross threads (6, 7, 9, 10, 11) are in the range of 100 to 110 threadsper cm in longitudinal direction.
 13. The forming screen according toclaim 1, characterized in that there are second cross threads (6, 7)which are only bound in the longitudinal thread layer (2) on the paperside.
 14. The forming screen according to claim 13, characterized inthat the second cross threads (6, 7), taken alone, form a plain weavewith the longitudinal thread layer (2) on the paper side.
 15. Theforming screen according to claim 1, characterized in that the bindingcross threads (10, 11) are arranged adjacent to one another to formgroups of at least two binding cross threads (10, 11) each, whereby oneof the binding cross threads (10, 11) binds in the longitudinal threadlayer on the machine side when the other binding cross thread (11, 10)or the other binding cross threads bind in the longitudinal thread layer(2) on the paper side.
 16. The forming screen according to claim 15,characterized in that the respectively adjacent binding cross threads(10, 11) bind in the longitudinal thread layer (2) on the paper sidewith their repeating staggered vis-à-vis one another in such a way thattheir binding patterns complement one another in transverse directionwithout a gap.
 17. The forming screen according to claim 15,characterized in that the binding cross threads (10, 11) bind in thelongitudinal thread layer (3) on the machine side within a repeatingwith less longitudinal threads (5) than in the longitudinal thread layer(2) on the paper side.
 18. The forming screen according to claim 17,characterized in that the binding cross threads (10, 11) bind in thelongitudinal thread layer (3) on the machine side within a repeatingwith not more than two longitudinal threads (5).
 19. The forming screenaccording to claim 1, characterized in that there are third crossthreads (8) which only bind in the longitudinal thread layer (3) on themachine side.
 20. The forming screen according to claim 19,characterized in that the third cross threads (8) float on the outerside of the longitudinal thread layer (3) on the machine side overseveral longitudinal threads (5), wherein the number of theselongitudinal threads (5) is always greater than the number of thelongitudinal threads (5) with which the third cross threads (8) bindbetween two floatings (9).
 21. The forming screen according to claim 20,characterized in that the third cross threads (8) float over more thanthree longitudinal threads (5) on the outside and, in between, only bindwith one longitudinal thread (5).